Device and method for locating anatomical cavity in a body

ABSTRACT

The invention describes a device ( 1 ) for locating an anatomical cavity ( 15 ) inside a body ( 10 ). The device comprises a fluid-filled reservoir ( 30 ) which is closed off in a sealed manner by a displaceable plunger ( 32 ) and is connected to a hollow puncture needle ( 20 ).  
     To measure the pressure prevailing in the fluid, the device comprises a pressure gauge ( 40 ). A signal converter ( 50 ) is used to convert a continuous pressure-measurement signal provided by the pressure gauge ( 40 ) into a form which is suitable for further processing. A synthesizer ( 60 ) is designed to process the converted pressure-measurement signal into a continuous sound signal which is representative of the pressure.  
     If, during a displacement of the puncture needle ( 20 ), a needle point ( 21 ) reaches the anatomical cavity ( 15 ), the result is a readily perceptible change in the sound signal.  
     The device comprises recording means for recording the pressure-measurement signal over the course of time.

[0001] The present invention relates to a device and a method forlocating an anatomical cavity in a body.

[0002] Locating an anatomical cavity in a body, for example the body ofa patient, is important, inter alia, for anaesthetics, in which it isoften necessary for substances which have an anaesthetic action to beintroduced into the anatomical cavity, such as for example the epiduralcavity which is located in the vicinity of the spinal cord. To locatethe anatomical cavity, it is generally known to use a hollow needle anda reservoir filled with an isotonic liquid or with a gas mixture, oftenan injection syringe with a displaceable plunger. In this case, thehollow needle is introduced into a patient's body, in the vicinity ofthe location where the cavity which is to be located is situated. Theinjection syringe is positioned at the free end of the needle, and theliquid or gas mixture can be injected from the injection syringe throughthe needle and can reach the body of the patient. The person who ishandling the injection syringe with the needle, for example a physician,uses one hand to introduce the needle further into the body and uses thethumb of the other hand to exert pressure on the plunger of theinjection syringe. The liquid will seek to flow out of the injectionsyringe via the open end of the needle, but in the process will besubject to a resistance from the tissue in which the point of the needleis situated. As a result, a certain force will have to be exerted on theplunger, and a pressure will be produced in the liquid. When the pointof the needle reaches the anatomical cavity, the liquid flowing out ofthe needle is no longer subject to any resistance from surroundingtissue, and the pressure in the liquid drops. The person who is handlingthe assembly can feel this in the hand which he is using to operate theinjection syringe. When the epidural cavity has been reached, a catheteris often introduced via the needle into the epidural cavity, so that,for example, an anaesthetic substance can be administered.

[0003] When the epidural cavity is being located, it is highly importantthat the point of the needle should not pass beyond this cavity, sincethere is then a risk of the spinal cavity or spinal cord behind it beingaffected, which may have particularly adverse results for the patient.Therefore, all kinds of methods and devices have been developed whichmake it easier to locate an anatomical cavity and reduce the risk of thepoint of the needle being introduced too far.

[0004] An example of a method and device of this type is known, interalia from EP 0 538 259.

[0005] The known device comprises a hollow needle, a fluid-filledreservoir which is in communication with the needle, pump means forpressurizing the fluid, measuring means for creating apressure-measurement signal which is related to the pressure prevailingin the fluid, signal-conversion means for converting thepressure-measurement signal created by the measuring means into a formwhich is suitable for further processing, and reproduction means foremitting an acoustic signal which is related to the pressure-measurementsignal which has been converted by the signal-conversion means.

[0006] The needle of the known device is intended to be introduced intoa body and is connected to the reservoir in the form of an injectionsyringe. The injection syringe contains a fluid in the form of anisotonic liquid. The needle and the injection syringe are incommunication with one another via a T-shaped connector. Thepressure-measuring means, which are used to detect and measure thepressure prevailing in the liquid in the injection syringe, are alsoconnected to this T-shaped connector. The known device also comprises aprocessor for processing a pressure-measurement signal created by thepressure-measuring means, in order that the rate of pressure variationcan always be determined, which pressure variation is primarily theconsequence of the movement of the needle in the body. The pressure dataprovided by the processor and the pressure-measuring means arecontinuously compared with margins stored in the processor.

[0007] When the known device is being used, the starting point is asituation in which the point of the needle is already situated in thevicinity of the cavity which is to be located. The needle, the injectionsyringe and the pressure-measuring means are connected to one another bychanging the position of a switch. In the injection syringe there is aplunger which functions as a pump means for displacing the liquidthrough the needle and thus produces pressure in the liquid. Thepressure which is shown on the screen when there is no pressure beingexerted on the plunger of the injection syringe is calibrated to zero.Then, the person who is handling the injection syringe and the needlebrings the pressure in the liquid in the injection syringe to a definedlevel by exerting pressure on the plunger. During this process, he canread the level of the pressure from the screen at any time. When theliquid in the injection syringe has been brought to the requiredpressure, the person who is handling the device can move the needletowards the cavity in the body while using the pressure data displayedon the screen to carefully maintain a pressure on the plunger. In theprocess, the pressure in the liquid will vary. When the level of thepressure variation exceeds a minimum level stored in the processorand/or the pressure variation rate over a defined time period is withinminimum margins stored in the processor, the warning means are activatedand emit a first, acoustic warning signal via acoustic reproductionmeans. If the pressure can be restored by slightly displacing theplunger, without further displacement of the needle, the first acousticwarning signal will stop. On the other hand, if a more abrupt pressurevariation occurs and the pressure cannot be restored by displacing theplunger, the warning means emit a second, acoustic warning signal, whichclearly differs from the first warning signal. From the second warningsignal, the person who is handling the device can infer that the pointof the needle has reached the anatomical cavity and that he must stopmoving the needle.

[0008] A drawback of the device and method which are known from EP 0 538259 is that the acoustic warning signals are emitted on the basis of aninterpretation of the pressure data by the processor. Although theperson who is handling the device can see the instantaneous pressure onthe screen and can also feel this information through the plunger, hewill quickly become inclined to depend only upon his hearing and trustthe acoustic warning signals. However, in practice this has not provensatisfactory, and consequently the person who is handling the devicewill also look at the screen, so that he loses sight of the needle.

[0009] It is an object of the present invention to provide an improveddevice for locating an anatomical cavity in which the above drawback iseliminated or at least reduced.

[0010] According to the present invention, the above object is achievedby providing a device for locating an anatomical cavity comprising ahollow needle and a reservoir which is in communication therewith and isfilled with a fluid. The reservoir is also connected to pump means forpressurizing the fluid. Furthermore, measuring means are provided forcreating a pressure-measurement signal which is related to the pressureprevailing in the fluid, and signal-conversion means are provided forconverting the pressure-measurement signal which has been created by themeasurement means into a form which is suitable for further processing.Acoustic reproduction means are also provided, which are designed toemit an acoustic signal which is representative of the pressureprevailing in the fluid.

[0011] In the device according to the invention, the pressure is madecontinuously available to the person who is handling the device and toany onlookers who may be present during the introduction of the needleinto the body by means of the acoustic signal, which is representativeof the prevailing pressure and therefore of the pressure signal. Theperson who is handling the device can therefore use his hearing toobtain instantaneous pressure data continuously, which data is directlyrepresentative of the pressure in the needle which is coupled to thereservoir. Consequently, the person who is handling the device cancontinuously direct his eyes at the needle and does not have to work onthe basis of visual information and/or tactile information. A particularadvantage is that the interpretation of the measured pressure is theresponsibility of the person who is handling the device, rather than ofa processor, and therefore it is possible to draw an interpretation withregard to intra-individual pressure variations for each case.

[0012] A further drawback of the device and method which are known fromEP 0 538 259 relates to the risk of the point of the needle beingintroduced too far. Although the known device reduces this risk, sincethe feeling of the person who is handling the device is assisted by thewarning signals, the risk of the needle shooting through (too far) whenthe anatomical cavity is reached as a result of one hand being used toexert pressure on the needle and the other hand being used to exertpressure on the plunger of the injection syringe is still present. Sincemoving the needle requires a relatively large amount of force, this riskis by no means negligible.

[0013] This drawback is at least partially overcome by an importantpreferred embodiment of the device according to the present invention,which is provided with automatic pump means and in particular isprovided with automatic pump means which are designed to continuouslydeliver the fluid.

[0014] When using the device according to the invention, the person whois handling the device continuously receives the instantaneous pressuredata on the basis of his hearing, and therefore there is no need to feelthe pressure in the fluid. The use of automatic pump means has the majoradvantage that the person who is handling the device can use both handsto move the point of the needle towards the anatomical cavity. When bothhands are being used, the person who is handling the device is able tointroduce the needle in a more controlled way than if only one hand isused for this purpose. Moreover, he does not now have to exert any forcein order to generate the pressure, but rather only has to exert force onthe needle.

[0015] In a further preferred embodiment of the device according to theinvention, the pump means comprise a displaceable plunger, and the pumpmeans are provided with a drive unit which is designed to displace theplunger in the reservoir, preferably at a constant rate.

[0016] The fact that the drive unit displaces the plunger, preferably ata constant rate, means that the force which is exerted on the plunger bythe automatic pump means will always have to vary, since the point ofthe needle is constantly moving through different types of tissue as itis being introduced. In the case of a relatively firm tissue, the fluidwhich emerges from the needle will be subject to more resistance fromthe tissue than in the case of a soft tissue. When the point of theneedle reaches the anatomical cavity, the fluid will flow into theanatomical cavity virtually without resistance, and therefore scarcelyany force has to be exerted on the plunger. This consequently results ina considerable drop in the pressure of the fluid.

[0017] The risk of the point of the needle being introduced too far isconsiderably reduced compared to the device which is known from EP 0 538259.

[0018] The device according to the invention is eminently suitable foruse for training purposes, in particular for training medical personnel.An instructor can easily monitor the actions of a student or trainee andprovide suitable instructions, since he can always hear informationabout the pressure state at the point of the needle. If the preferredembodiment with the drive means is being used, it has the additionaladvantage that the student or trainee only has to concentrate on themovement of the needle and can use both hands to do this.

[0019] The present invention also relates to a measuring and signallingdevice which is intended in particular to be used in the device forlocating an anatomical cavity, and to a method for measuring thepressure in a fluid at the location of the point of a hollow needlewhich is situated in a body for the benefit of an observer.

[0020] These and other aspects, characteristics and advantages of thepresent invention will be explained in more detail by means of thefollowing description of two preferred embodiments of a locating deviceaccording to the invention with reference to the drawing, in whichidentical reference numerals denote identical components, and in which:

[0021]FIG. 1 shows a side view of a first preferred embodiment of thedevice according to the invention, in which a section of the device isillustrated diagrammatically, and

[0022]FIG. 2 diagrammatically depicts a section of a second preferredembodiment of the device according to the invention.

[0023]FIG. 1 shows a first preferred embodiment of the device accordingto the invention, which is denoted overall by reference numeral 1.

[0024]FIG. 1 shows one of the possible uses of the device 1,specifically its use for locating an anatomical cavity, in particularthe epidural cavity, in a human body. A small section of a human body isillustrated in cross section in FIG. 1 and is denoted by referencenumeral 10. The anatomical cavity or epidural cavity is denoted byreference numeral 15.

[0025] The device 1 comprises a hollow puncture needle 20 with a needlepoint 21. In the example shown in FIG. 1, the puncture needle 20 hasbeen introduced part way into the body 10 and the needle point 21 issituated in the vicinity of the epidural cavity 15. The puncture needle20 is provided with a handle 22, which can be held by the person who ishandling the puncture needle 20, for example a physician. This handle 22enables the user to exert a pushing or pulling force on the punctureneedle 20, in order for it to be moved in a desired direction.

[0026] A reservoir 30 in the form of an injection syringe is positionedin line with the puncture needle 20. The reservoir 30 comprises afluid-filled space 31 and a displaceable plunger 32 which closes off thesaid reservoir 30 in a sealed manner on one side. The displaceableplunger 32 in this case functions as a pump means for generatingpressure in the fluid in the reservoir 30. As an alternative to using aplunger as the pump means, it is also possible, for example, to usebellows or other suitable pump means. The reservoir 30 is incommunication with the puncture needle 20 via a connection part 33, itbeing possible for fluid to flow out of the said reservoir 30 into thesaid puncture needle 20. The fluid in the reservoir 30 may be a gas or aliquid, for example a sterile, isotonic liquid. In the text whichfollows, the invention is discussed on the basis of the exampleaccording to which an isotonic liquid is situated in the reservoir 30,but the invention can equally well be applied to an embodiment in whicha gas is situated in the reservoir 30.

[0027] To measure the pressure in the liquid in the reservoir 30, thelocating device 1 is provided with an electrical pressure gauge 40. Anoutput 41 of the pressure gauge 40 is connected to an input 51 of asignal converter 50, which is able to convert a pressure-measurementsignal provided by the pressure gauge 40 into a form which can be usedfor further processing, for example an electric voltage. The signalconverter 50 is designed to emit the converted pressure-measurementsignal both at an acoustic output 52 and at a visual output 53. Itshould be noted that the pressure gauge 40 and the signal converter 50can form a single unit instead of two separate elements.

[0028] One input 61 of a synthesizer 60 is connected to the acousticoutput 52. The synthesizer 60 is designed to process the convertedpressure-measurement signal to form a sound signal which isrepresentative of the pressure prevailing in the reservoir 30 andtherefore of the said converted pressure-measurement signal. Thesynthesizer 60 is connected via an output 62 to a loudspeaker 70 foremitting the sound signal. If appropriate, an audio amplifier (not shownin more detail) is also provided, for the purpose of amplifying thesound signal.

[0029] On account of the relationship between the sound signal and theconverted pressure-measurement signal, the sound signal isrepresentative of the pressure which is prevailing in the liquid in thereservoir 30. One or more aspects of the sound signal may be directlyand continuously related to the converted pressure-measurement signal,depending on the setting of synthesizer 60. For example, the pitch ofthe sound signal may be representative of the pressure, but it is alsopossible, for example, that the volume or the pulse frequency, if thesound signal has a pulsating character, may be representative of thepressure. Another possibility is for a suitable combination of thepitch, the volume and the pulse frequency to be used. One possiblecombination which may be suitable is, for example, the use of the pitchas a measure of the pressure and the volume as a measure of the rate atwhich the pressure is changing, in which case, by way of example, in theevent of a rapid pressure change a stronger sound signal is emitted thanin the event of a slow change. In this example, another possibility isto swap the pitch and volume.

[0030] An input 81 of an amplifier 80 is connected to the visual output53 and is designed to emit an amplified, converted pressure-measurementsignal at an output 82. The amplifier 80 is connected via the output 82to a screen 90 for displaying a visual signal which is representative ofthe pressure in the liquid in the reservoir 30 on the basis of theamplified, converted pressure-measurement signal.

[0031] The assembly comprising pressure gauge 40, signal converter 50,synthesizer 60, loudspeaker 70, amplifier 80 and screen 90 is referredto below as the measuring and reproduction unit 100, as shown by adashed line in FIG. 1.

[0032] Before the device 1 can be used to locate the epidural cavity 15in the body 10, the puncture needle 20 can first of all be introduced asmall depth into the body 10 in the vicinity of where the epiduralcavity is situated. Then, the reservoir 30 and the measuring andreproduction unit 100 coupled thereto can be connected to the punctureneedle 20. It is also possible for all the components to be connected toone another first of all and then for the liquid to be pressurized, andfor the puncture needle 21 to be introduced into the body 10 as soon asthe liquid emerges from the needle 20. The latter method is preferred,since in this case the measurement of the pressure can commenceimmediately.

[0033] If the person who is handling the device 1 exerts force on theplunger 32, pressure will be built up in the liquid which is situated inthe reservoir 30. As a result of this pressure, the liquid will tend toflow out of the unit comprising reservoir 30 and puncture needle 20 viathe needle point 21. In the process, the liquid which emerges at theneedle point 21 is subject to a resistance to absorption of the liquidby the tissue in which the needle point 21 is situated. This resistanceis expressed as a pressure which has to be overcome by the person who ishandling the device 1 by exerting a force on the plunger 32. If theperson who is handling the device 1 always ensures a (slight)displacement of the plunger 32 while he is moving the puncture needle 20towards the epidural cavity 15, the force exerted on the plunger 32, andtherefore the pressure prevailing in the liquid, is always a measure ofthe pressure at the needle point 21.

[0034] The synthesizer 60 is set in such a manner that the sound signalwhich is emitted is representative of the measured pressure, with theresult that the sound signal is always an acoustic reproduction of theinstantaneous pressure in the liquid.

[0035] Any changes in the resistance which the liquid experiences as itleaves the needle point 21 have a direct influence on the pressure inthe reservoir 30. The changes in the pressure in the reservoir 30 inturn directly lead to changes in the sound signal. The person who ishandling the puncture needle 20 can therefore use any changes in thesound signal which he detects over the course of time to determine whatchanges are occurring in the pressure. If the possibility of a visualreproduction is being used, the same-applies to the visual signal.

[0036] When the epidural cavity 15 has been reached by the needle point21, the liquid which emerges will be subject to considerably lessresistance. As a result, less force has to be exerted on the plunger 32in order to displace the liquid out of the reservoir 30. As a result,the pressure in the reservoir 30 will drop. With this knowledge, it canbe inferred from a sudden and significant change in the sound signalthat the needle point 21 has reached the epidural cavity 15 and thatmovement of the puncture needle 20 into the body 10 can be stopped. Ifthe possibility of a visual reproduction is being-used, the person whois handling the device 1 can look at the screen 90 and see confirmationthat the epidural cavity 15 has been reached.

[0037] When using the device 1, calibration of the pressure-measurementsignal is not critical, since the user is working on the basis ofchanges which he detects in the sound signal; the absolute value of, forexample, the volume or pitch of the sound signal is of subordinateimportance.

[0038] The continuous pressure-measurement signal is likewise fed by thesignal converter 50, as a voltage signal, via the visual output 53 tothe input 81 of the amplifier 80. Via the output 82, thepressure-measurement signal is supplied in amplified form to the screen90, which emits a visual signal on the basis of the pressure-measurementsignal which is supplied. In this case too, there is a relationshipbetween the pressure-measurement signal and the visual signal, thevisual signal being representative of the pressure at the needle point21.

[0039] The screen 90 may, for example, be designed to reproduce thevisual signal in the form of numerical values or plotted in the form ofa curve of the measured pressure against time. The visual reproductionin the form of a curve of the pressure plotted against time isparticularly preferred, since it has been found that reaching theepidural cavity 15 results in a highly characteristic and thereforehighly recognisable curve. The screen 90 may be provided with meanswhich allow the numerical values to be calibrated, but this is notimperative, since the user can, in the case of the visual signal aswell, work on the basis of changes in said signal to determine whetheror not the needle point 21 has reached the epidural cavity 15.

[0040] During the introduction of the needle 20 into the body, thecontinuous sound signal is of greater importance than the continuousvisual signal. This is because the user cannot continuously look at thescreen 90 in order to observe the visual signal, since he must also atleast look at the puncture needle 20 during the movement thereof. Tohandle the device 1, the user can in principle rely entirely on thesound signal and use his eyes to look at the movement of the punctureneedle 20.

[0041] The sound signal can provide the user with sufficient informationwith regard to the pressure at the needle point 21. Therefore, in thedevice 1 according to the invention, the visual output 53 on the signalconverter 50, the amplifier 80 and the screen 90 can be omitted ifdesired, and therefore the device 1 according to the invention providedwith the visual output 53 at the signal converter 50, the amplifier 80and the screen 90 forms a particular variant of the first preferredembodiment. However, as has been mentioned, the visual signal does offera monitoring option.

[0042]FIG. 2 diagrammatically depicts a section of a second preferredembodiment of a device 2 according to the invention. The secondembodiment of the device 2 comprises the same components as the firstpreferred embodiment shown in FIG. 1.

[0043] The hollow puncture needle 20 (cf. FIG. 1) which forms part ofthe device 2 is not shown in FIG. 2. The reservoir 30 is connected tothe puncture needle by means of a hose 34 and the connection part 33.

[0044] The device 2 is provided with automatic pump means which aredesigned to displace the fluid, and in particular the plunger 32 and theliquid situated beneath it (or if desired a gas mixture) in thereservoir 31 as a result of a force being exerted on the plunger 32. Forthis purpose, the device 2 comprises a connecting rod 35, one end ofwhich is connected to a drive unit 110 and the other end of which isprovided with a pressure disc 36. The drive unit 110 is used to exert aforce on the connecting rod 35 in order to effect a movement in theaxial direction of the connecting rod 35. The pressure disc 36 isintended to bear against at least part of the surface of the plunger 32.By way of example, the drive unit 110 may be an electric motor which isconnected to a (storage) battery 120. An example of a suitable driveunit for the preferably automatic displacement of the plunger 32 is aninjection pump which is known in the specialist field. A pump of thistype is particularly suitable for delivering small quantities of liquidwith a high degree of accuracy. Other possibilities are conceivable forsupplying energy to the drive unit 110. For example, the device 2 couldbe provided with means for connecting the drive unit 110 to the mains.It is also possible to arrange a pump between the reservoir 30 and theneedle 20. In this case, the pump means do not generate any pressure inthe reservoir, but rather suck the liquid out of the reservoir and pumpit through the needle under pressure. Therefore, the measurement of thepressure in the reservoir as described above can no longer take place,but rather will have to take place either in the needle or immediatelyafter the pump. Another possibility for determining the pressuregenerated is that of measuring the power consumed by the pump means andusing this information to calculate the pressure difference which hasbeen generated by the pump means. This possibility is not shown in moredetail.

[0045] A significant advantage of the automatic displacement of theplunger 32 is that the person who is using the device 2 can use bothhands to move the puncture needle. As has already been stated in thedescription of the first preferred embodiment of the device (FIG. 1),the pressure in the reservoir 30 is continuously rendered audible bymeans of an acoustic signal. Consequently, it is not necessary for theperson who is handling the device to exert pressure on the plunger 32himself. However, this is required, for example, when the device whichis known from EP 0 538 259 is being used.

[0046] The reservoir 30, the connecting rod 35, the drive unit 110 andthe (storage) battery 120 are all arranged in the interior of a housing130, as is the measuring and reproduction unit 100, which isdiagrammatically indicated by a dashed line in FIG. 2. An arrangement ofthis type is preferred, since it results in a compact assembly which caneasily be connected to the puncture needle.

[0047] The reservoir 30 is accommodated in a space which is created forthis purpose in the housing 130, which at the location of the said spaceis preferably designed in such a manner that a reservoir 30 can easilybe placed into the housing 130 before use and can easily be removedagain from the housing 130 after use.

[0048] Clamping means (not shown) may be provided on the exterior of thehousing 130, in order to allow the said housing 130 to be clamped to anydesired object, such as for example a table or a bed.

[0049] In this example, the pressure gauge 40 comprises a force pick-upwhich is designed to pick up the force exerted on the connecting rod 35by the drive unit 110. In this case, the signal converter 50 is designedas a force-voltage converter. The drive unit 110 comprises a controlunit 111 which is connected to the force pick-up of the pressure gauge40. The control device 111 is responsible for displacement of theplunger 32 at a continuous rate and therefore for producing a constantmass flow of the liquid through the needle. As a result of thedisplacement of the plunger 32, pressure will be built up in the liquidwhich is situated in the reservoir 30. As a result of this pressure, theliquid will tend to flow out of the assembly comprising reservoir 30 andpuncture needle via the needle point. In the process, the liquid whichemerges at the needle point is subject to a resistance to absorption ofthe liquid by the tissue in which the needle point is situated. Thisresistance manifests itself as a pressure which has to be overcome bythe drive unit 110 as a result of a force being exerted on the plunger32. The continuous but slow rate at which the plunger 32 is beingdisplaced while the puncture needle is being moved in the direction ofthe epidural cavity means that the force exerted on the plunger 32, andtherefore the pressure prevailing in the liquid, will always be ameasure of the pressure at the needle point.

[0050] Although it is preferred for the plunger 32 to be moved at aconstant rate, it is also possible for the plunger 32 to be moved insteps. In this case, however, it is preferable for the movement steps tofollow one another at sufficiently short intervals for the risk of theneedle being introduced too far to be avoided.

[0051] The force with which the drive unit 110 presses on the plunger 32is controlled by the control device 111. For safety reasons, the controldevice 111 may be provided with a pressure limiter, which limits themaximum pressure prevailing in the liquid and therefore the maximumforce which can be exerted on the plunger 32. This prevents thepossibility of force being exerted on the plunger 32 in an uncontrolledmanner and ultimately of the body being damaged if liquid is forced intothe body with considerable force.

[0052] For suitable driving, a section of the connecting rod 35 may beprovided with helical toothing, in which case the drive unit 110 isprovided with a driven toothed wheel which meshes with the said section.The force pick-up of the pressure gauge 40 may in this case be designedto measure the force exerted on the connecting rod 35 by the wheel.

[0053] The process of locating the epidural cavity begins withpressurizing an isotonic liquid, for example, in the reservoir 30 bymeans of a displacement of the plunger 32 in the direction of the hose34. For this purpose, the drive unit 110 exerts a force, which isdirected to the left in FIG. 2, on the connecting rod 35. As a result ofthis force, the connecting rod 35 will move in the axial direction andthe plunger 32 will likewise move in the direction of the hose 34, viathe pressure disc 36. In the process, the liquid tends to move towardsthe body and in the process is subject, as has already been discussed,to a resistance from the tissue of the body, which is experienced as acounterpressure which is exerted by the body and is translated into apressure in the liquid.

[0054] The force which is exerted on the connecting rod 35 by the driveunit 110 in order to inject the liquid into the tissue of the body isdirectly related to the pressure at the needle point and must overcomeat least the said resistance. If the needle point is moving throughsubstantially homogeneous tissue, the pressure at the needle pointscarcely changes and the force which is to be exerted is virtuallyconstant. The amplifier 70 will in this case emit a sound signal whichlikewise scarcely changes.

[0055] The drive unit 110 is designed to move the connecting rod 35 at aconstant, preferably low rate after the liquid in the reservoir 30 hasbeen brought to the said starting pressure. As the tissue will notalways present the same resistance to absorption of the liquid, theforce with which the drive unit 110 presses on the plunger 32 willchange when the needle point is then being moved through the tissue ofthe body, since a constant movement/movement rate of the plunger 32 isdesired. As a result, the pressure in the liquid will likewise changeduring the movement of the needle point. The measuring and reproductionunit 100, as has been described with reference to FIG. 1, measures thepressure and supplies an acoustic and/or visual signal.

[0056] When the needle point reaches the epidural cavity, the liquidflows into the epidural cavity and the resistance to absorption of theliquid will drop suddenly and significantly. As a result, the pressureprevailing in the liquid will likewise drop suddenly and significantly.At that time, the drive unit 110 has to exert in relative terms a muchlower force in order to move the connecting rod 35 than was the casebefore the epidural cavity was reached. The sudden and significantchange in the force is observed by the person who is handling the device2 as a sudden and significant change in the sound signal and also, ifappropriate, in the visual signal. The person who is handling the device2 can infer from a change of this type that the needle point has reachedthe epidural cavity and that he must stop moving the puncture needle. Hecan then disconnect the connection between the puncture needle and theremaining components of the device at the connection part 33, in orderfor an injection syringe containing the substances which are to beintroduced into the epidural cavity to be placed on to the punctureneedle or in order to introduce what is known as an epidural catheter.

[0057] In an embodiment which is not shown in more detail, the deviceaccording to the invention comprises recording means for recording theprofile of the pressure-measurement signal over the course of time. Inparticular, consideration may be given to an electronic memory in whichthe measured pressure data are stored and can subsequently be retrieved,or, for example, a magnetizable storage means. However, it is alsopossible for the profile of the pressure-measurement signal to beprinted directly onto paper.

[0058] A major advantage of recording means of this type is that it ispossible to retrospectively assess whether, and if necessary todemonstrate that, the person who is handling the device did in factreach the intended anatomical cavity. This advantage is all the greaterif the recorded pressure data are stored during the movement of theneedle in the body in such a way that, with this data, it is easy toproduce a curve in which the measured pressure is plotted against time.As has already been noted above, a visual reproduction in the form of acurve of the pressure plotted against time is highly advantageous, sincereaching the anatomical cavity produces a very characteristic andtherefore very recognisable curve. It should be noted that recordingmeans for storing pressure-measurement data may also be used separatelyof the inventive idea. The recording means do not necessarily have to beused in combination with reproduction means for generating an acousticor visual signal. The storage is important for subsequent reference tothe pressure-measurement data.

[0059] The scope of the present invention is not restricted to theexamples which have been discussed above, but rather various changes andmodifications to these examples are possible without departing from thescope of the invention as defined in the appended claims.

[0060] For example, it is also possible for the inventive idea to beapplied to a step which often follows the step of locating an epiduralcavity, namely the positioning of an epidural catheter in the epiduralcavity.

[0061] The locating of the epidural cavity is often a preparatory stepto administering an anaesthetizing substance via the said catheter. Inthis case, it is important for the end of the catheter to be located atthe correct location, i.e. in the epidural cavity.

[0062] The catheter can be introduced by disconnecting the reservoirfrom the puncture needle after the needle point has reached the epiduralcavity and then guiding the catheter via the puncture needle into theepidural cavity. Often, the person who is positioning the catheter willhave a reasonable feeling of when the catheter has been correctlypositioned, but more certainty with regard to correct positioning isdesirable. The measuring and signalling device according to the presentinvention can advantageously be used in this context.

[0063] When the catheter has been introduced via the puncture needle,its correct positioning can be checked by pressurizing the liquid whichis to be introduced and measuring the pressure, and in particular thechange(s) in this pressure, using the measuring and signalling deviceaccording to any desired preferred embodiment of the measuring andsignalling device.

[0064] While the liquid in the catheter is being pressurized, which isthe result of the liquid being fed to the catheter, the pressure of theliquid will initially increase. If the liquid is then pumped through thecatheter and if the end of the catheter is in fact located in theepidural cavity, the liquid will be able to flow virtually unimpededinto the epidural cavity, the pressure in the liquid will therefore notchange or will scarcely change, and there will be virtually no change inthe acoustic signal and/or the visual signal which is generated by themeasuring and signalling device. The person who is carrying out theoperation will immediately know that the catheter has been positionedcorrectly and will be able to fix the catheter in place.

[0065] The above text has described an application of the deviceaccording to the invention for locating an epidural cavity in a humanbody. This does not detract from the fact that the device can also beused to locate a region which is situated in a body and in terms of itsproperties differs from the area which immediately surrounds it. In thiscontext, consideration may be given, for example, to locating anintumescence or a tumour in the body of a person. A tumour generally hasdifferent properties from the surrounding tissue in which it issituated, and in particular the tumour will present a differentresistance to the penetration of a fluid compared to the surroundingtissue. If the device according to the invention is used in a similarway to that described above, a sudden change in the measured pressure atthe needle point will indicate that the area which is being looked forhas been reached. In this context, it is assumed that pressurevariations which occur in the fluid during displacement of the needle ina section of the body which precedes the area which is to be located aremuch smaller than the pressure variation which occurs in the fluid whenthe needle point reaches the said area. It is advantageous if it isknown how the area which is to be located differs from the part whichsurrounds this area, so that it is possible to establish with a highdegree of certainty that the area which is to be located has in factbeen found.

[0066] The device can also be used to locate any desired cavity in anydesired body, once again assuming that pressure variations which occurin the fluid during displacement of the puncture needle in a section ofthe body which precedes the cavity are significantly smaller than thepressure variation which occurs in the fluid when the needle pointreaches the cavity.

[0067] Furthermore, the above text has described the device according tothe invention using an isotonic liquid. As has already been noted above,it is also possible to use a gas. However, a gas is a medium which canbe compressed to a much greater extent than a liquid. To compensate forthis while the device is being used and to ensure a constant mass flowof the gas during use of the device, the device may be provided withadditional control means.

What is claimed is:
 1. Device for locating a region which is situated ina body, comprising: a hollow needle; a fluid-filled reservoirs which isin communication with said needle: pump means for pressurizing thefluid; measuring means for creating a pressure-measurement signal whichis related to the pressure prevailing in the fluid; signal-conversionmeans for converting said pressure-measurement signal created by saidmeasuring means into a form which is suitable for further processing,and reproduction means for emitting an acoustic signal which is relatedto said pressure-measurement signal which has been converted by saidsignal-conversion means, wherein said reproduction means are designed toemit said acoustic signal which is representative of the pressureprevailing in the fluid.
 2. Device according to claim 1, herein thepitch of said acoustic signal is representative of the pressureprevailing in the fluid.
 3. Device according to claim 1, wherein thevolume of said acoustic signal is representative of the pressureprevailing in the fluid.
 4. Device according to claim 1, wherein saidsignal-conversion means are designed to emit said pressure-measurementsignal in the form of an electric voltage.
 5. Device according to claim1, wherein said reproduction means comprise a synthesizer, an audioamplifier and a loudspeaker.
 6. Device according to claim 1,characterized by visual reproduction means which are designed to emit avisual signal which is representative of the pressure prevailing in thefluid.
 7. Device according to claim 6, wherein the visual reproductionmeans comprise an amplifier and a screen.
 8. Device according to claim1, characterized by recording means which are designed to record theprofile of said pressure-measurement signal over the course of time. 9.Device according to claim 1, wherein said pump means comprise adisplaceable plunger.
 10. Device according to claim 9, characterized byautomatic pump means.
 11. Device according to claim 10, wherein saidautomatic pump means are designed to continuously deliver fluid. 12.Device according to claim 10, wherein said pump means comprise a driveunit which is designed to displace said plunger in said reservoir. 13.Device according to claim 12, wherein said measuring means are designedto pick up the force which is exerted on said plunger by said driveunit.
 14. Device according to claim 1, wherein pressure-limiting meansare provided for the purpose of limiting the fluid pressure. 15.Measuring and signalling device, intended in particular for use in adevice according to claim 1, comprising: a housing; a space which islocated in the interior of said housing and is suitable foraccommodating a fluid-filled reservoir which is connected to pump meansfor pressurizing the fluid, said reservoir being designed to beconnected to a hollow needle; measuring means for creating apressure-measurement signal which is related to the pressure prevailingin the fluid; signal-conversion means for converting saidpressure-measurement signal created by said measuring means into a formwhich is suitable for further processing; and reproduction means whichare designed to emit an acoustic signal which is representative of saidpressure-measurement signal which has been converted by saidsignal-conversion means.
 16. Measuring and signalling device accordingto claim 15, wherein said pump means comprise a displaceable plunger.17. Measuring and signalling device according to claim 16, whereinautomatic pump means are provided.
 18. Measuring and signalling deviceaccording to claim 17, wherein said pump means are designed to deliverfluid continuously.
 19. Measuring and signalling device according toclaim 17, wherein said pump means comprise a drive unit for displacingsaid plunger.
 20. Measuring and signalling device according to claim 19,wherein said measuring means are designed to create saidpressure-measurement signal on the basis of the force exerted on saidplunger by said drive unit.
 21. Measuring and signalling deviceaccording to claim 19, wherein said pump means are provided with adisplaceable connecting rod, one end of which is intended to bearagainst said displaceable plunger, and with a drive unit for effecting adisplacement of said connecting rod.
 22. Measuring and signalling deviceaccording to claim 21, wherein said measuring means are provided with aforce pick-up for picking up the force exerted on said connecting rod.23. Measuring and signalling device according to claim 22, wherein saidsignal-conversion means comprise a force-voltage converter. 24.Measuring and signalling device according to claim 21, wherein at leasta section of said connecting rode is provided with helical toothing. 25.Measuring and signalling device according to claim 15, wherein saidacoustic reproduction means comprise a synthesizer and a loudspeaker.26. Measuring and signalling device according to claim 15, characterizedby visual reproduction means for emitting a visual signal which isrepresentative of said pressure-measurement signal.
 27. Measuring andsignalling device according to claim 26, wherein said visualreproduction means comprise an amplifier and a screen.
 28. Measuring andsignalling device according to claim 15 characterized by recording meanswhich are designed to record the profile of said pressure-measurementsignal over the course of time.
 29. Measuring and signalling deviceaccording to claim 15, characterized by clamping means which areprovided on the exterior of the housing for clamping said device to anydesired object.
 30. Device for locating a region which is situated in abody, comprising: a hollow needle; a fluid-filled reservoir which is incommunication said needle; pump means for pressurizing the fluid;measuring means for creating a pressure-measurement signal which isrelated to the pressure prevailing in the fluid; signal-conversion meansfor converting said pressure-measurement signal created by saidmeasuring means into a form which is suitable for further processing,characterized by recording means which are designed to record saidpressure-measurement signal over the course of time.
 31. Deviceaccording to claim 30, wherein said recording means comprise anelectronic memory which is designed to store said pressure-measurementdata.
 32. Device according to claim 30, wherein said recording meanscomprise a magnetizable storage means, which is designed to store saidpressure-measurement data.
 33. Method for signalling pressure in a bodyat the location of a needle point of a hollow needle located in saidbody to an observer, comprising the following steps: introducing fluidinto the needle: applying an elevated pressure to the fluid; measuringthe pressure in the fluid; and generating an acoustic signal which isrepresentative of the instantaneous pressure in the fluid.
 34. Methodaccording to claim 33, characterized by the generation of a visualsignal which is representative of said instantaneous pressure in thefluid.
 35. Assembly for a device according to claim 1, comprising:measuring means for creating a pressure-measurement signal which isrelated to the pressure prevailing in the fluid; signal-conversion meansfor converting said pressure-measurement signal created by saidmeasuring means into a form which is suitable for further processing;and reproduction means which are designed to emit an acoustic signalwhich is representative of said pressure-measurement signal which hasbeen converted by said signal-conversion means.